Influenza viruses are constantly evolving, in fact all influenza viruses undergo genetic changes over time. The information CDC collects from studying genetic changes (also known as “substitutions,” “variants” or “mutations”) in influenza viruses plays an important public health role by helping to determine whether existing vaccines and medical countermeasures (e.g., antiviral drugs) will work against new influenza viruses, as well as helping to determine the potential for influenza viruses in animals to infect humans.